Chemical-Looping Combustion with Liquid Fuel
Doctoral thesis, 2014
Chemical-looping combustion (CLC) is a promising technology for future energy conversion on the basis of combustion of fossil fuels with inherent CO2 separation. In comparison to other carbon-capture technologies there is no need for energy demanding gas separation in order to obtain pure CO2. In the past, CLC research has mainly focused on the use of gaseous and solid fuels. This work the first comprehensive study of the use of liquid fuels in chemical-looping combustion. The work in the thesis includes (1) investigation of several different oxygen carriers based on nickel, iron, manganese and copper with kerosene as fuel, (2) design of an injection system for heavy liquid fuels for the fuel reactor of a pilot-scale chemical-looping reactor, (3) long-term operation of ilmenite oxygen carrier with a fuel oil in a pilot-scale unit and (4) tests with ilmenite oxygen carrier and a vacuum residue from Saudia Arabia as fuel.
Experiments were conducted in a bench-scale reactor and in a pilot-scale reactor, and for each reactor a fuel injection system was designed and tested. Two types of kerosene, with and without sulphur, were tested in the bench-scale reactor during 240 h of fuel injection and a fuel input between 100 Wth and 580 Wth. In the pilot-scale system, a fuel oil type 1 and different blends of a vacuum residue and fuel oil 1 were tested during 72 h of fuel operation. Here, the fuel input was varied between 4.5 kWth and about 6 kWth.
Six different oxygen-carrier materials, out of which four were synthesized and two were of mineral origin, were tested in the bench-scale unit. The different materials showed clear differences in fuel conversion and structural integrity. Ilmenite oxygen carrier, which is a mineral iron-titanium oxide, was found to have a high structural stability, which was unmatched by the synthesized materials. Fuel conversion, however, was significantly lower than that of the synthesized oxygen carriers, and it is likely that temperatures above 950°C are needed in order to achieve reasonable conversion levels.
Long-term testing was carried out with the ilmenite oxygen carried in the pilot-scale reactor system. The conditions in this unit are much harsher than those in the bench-scale reactor and similar to those in an industrial-scale circulating fluidized-bed boiler. With fuel oil 1 as fuel, different trends in fuel conversion could be observed with respect to reactor temperature, fuel flow and solids circulation. After modifying the fuel injection system, two different blends of vacuum residue and fuel oil were injected. At most, 80 wt% vacuum residue was blended with 20 wt% fuel oil. It was found that the conversion of the different blends tested did not deviate significantly from the conversion of unblended fuel oil. The total fuel operation time in the pilot-scale unit was about 72 h and the ilmenite particles were fluidized at high temperatures for more than 340 h without any significant problems concerning agglomeration or disintegration.